Required NPSH

It is difficult to determine exactly the areas of localized pressure reductions inside the pump, although much research has been focused on this field. It is easy, however, to measure the total fluid pressure (static plus dynamic) at some convenient point, such as pump inlet flange, and adjust it in reference to the pump centerline location. By testing, it is possible to determine the point when the pump loses performance appreciably, such as 3% head drop, and to define the NPSH at that point, which is referred to as a required NPSH (NPSHR). The available NPSH (NPSHA) indicates how much suction head... 

It is the energy in the liquid rec]uired to overcome the friction los.ses from the suction nozzle to the eye of the impeller without causing vaporization. It is a characteristic of the pump and is indicated on the pump s curve. It varies by design, size, and the operating conditions. It is determined by a lift test, producing a negative pressure in inches of mercury and converted into feet of required NPSH. 

This available value of NPSHa (of the system) must always be greater b) a minimum of two feet and preferably three or more feet than the required NPSH stated by the pump manufacturer or shown on the pump curves in order to overcome the pump s internal hydraulic loss and the point of lowest pressure in the eye of the impeller. The NPSH required by the pump is a function of the physical dimensions of casing, speed, specific speed, and type of impeller, and must be satisfied for proper pump performance. The pump manufacturer must ahvays be given complete Suction conditions if he is to be expected to recommend a pump to give long and trouble-free service. 

The pump selected for this application (water boiling at 0.98 psia) must have a required NPSH less than 4.7 ft, preferably about 3 to 3.5 ft. This is a difficult condition. If possible the vessel should be elevated to make more head (S) available, which wll raise the available NPSH. 

For hydrocarbons and water significantly above room temperatures, the Hydraulic Insdtute [17] recommends the use of a correction deduction as given in Figure 3-46. This indicates that the required NPSH as given on the pump curves can be reduced for conditions within the range of the curve based on test data. 

Fang, Ken-Shou, How Accurate are Predictions of Required NPSH Wren Based on Speed-Scale Effect , Power, V. 123, No. 2, 1979. 

To prevent cavitation, it is necessary that the pressure at the pump suction be sufficiently high that the minimum pressure anywhere in the pump will be above the vapor pressure. This required minimum suction pressure (in excess of the vapor pressure) depends upon the pump design, impeller size and speed, and flow rate and is called the minimum required net positive suction head (NPSH). Values of the minimum required NPSH for the pump in Fig. 8-2 are shown as dashed lines. The NPSH is almost independent of impeller diameter at low flow rates and increases with flow rate as well as with impeller diameter at higher flow rates. A distinction is sometimes made between the minimum NPSH required to prevent cavitation (sometimes termed the NPSHR) and the actual head (e.g., pressure) available at the pump suction (NPSHA). A pump will not cavitate if NPSHA > (NPSHR + vapor pressure head). 

The minimum required NPSH on the pump curves is normally determined using water at 60°F with the discharge line fully open. However, even though a pump will run with a closed discharge line with no bypass, there will be much more recirculation within the pump if this occurs, which increases local turbulence and local velocities as well as dissipative heating, both of which increase the minimum required NPSH. This is especially true with high efficiency pumps, which have close clearances between the impeller and pump casing. 

Water at 160°F is to be pumped at a rate of 100 gpm through a 2 in. sch 80 steel pipe from one tank to another located 100 ft directly above the first. The pressure in the lower tank is 1 atm. If the pump to be used has a required NPSH of 6 ft of head, what is the maximum distance above the lower tank that the pump may be located ... 

A liquid with a viscosity of 5cP, density of 45 lbm/ft3, and vapor pressure of 20 psia is transported from a storage tank in which the pressure is 30 psia to an open tank 500 ft downstream, at a rate of 100 gpm. The liquid level in the storage tank is 30 ft above the pump, and the pipeline is 2 in. sch 40 commercial steel. If the transfer pump has a required NPSH of 15 ft, how far downstream from the storage tank can the pump be located without danger of cavitation ... 

You must chose a centrifugal pump to pump a coal slurry. You have determined that the pump must deliver 200 gpm at a pressure of at least 35 psi. Given the pump characteristic curves in Appendix H, tell which pump you would specify (give pump size, speed, and impeller diameter) and why What is the efficiency of this pump at its operating point, what horsepower motor would be required to drive the pump, and what is the required NPSH of the pump The specific gravity of the slurry is 1.35. 

The available NPSH given by equations 4.8 and 4.9 must exceed the value required by the pump and specified by the manufacturer. The required NPSH increases with increasing flow rate as discussed below. 

A centrifugal pump will operate normally at a point on its total head against capacity characteristic curve until the available NPSH falls below the required NPSH curve. Beyond this point, the total head generated by a centrifugal pump falls drastically as shown in Figure 4.6 as the pump begins to operate in cavitation conditions. 

For quick pump selection, manufacturers often give the most essential performance details for a whole range of pump sizes. Figure 10-40 shows typical performance data for a range of process pumps based on suction and discharge pipes and impeller diameters. The performance data consists of pump flow rate and head. Once a pump meets a required specification, then a more detailed performance data for the particular pump can be easily found based on the curve reference number. Figure 10-41 shows a more detailed pump performance curve that includes, in addition to pump head and flow, the brake horsepower required, NPSH required, number of vanes, and pump efficiency for a range of impeller diameters. 

The conversion of the pump s suction pressure to velocity in the eye of the impeller is called the required net positive suction head (NPSH). As the flow-control valve on the discharge of the pump shown in Fig. 25.1 is opened, the velocity of liquid in the eye of the impeller goes up. More of the pump s suction pressure, or feet of head, is converted to velocity, or kinetic energy. This means that the required NPSH of a pump increases as the volumetric flow through the pump increases. 

The units of NPSH are feet of liquid head. The required NPSH of a pump is due primarily to the conversion of feet of head to velocity in the eye of the impeller. 

When the required NPSH of a pump equals the NPSH available to the pump, the pump will cavitate. 

The required NPSH of the pump may be read from Fig. 25.2 (regardless of the SG of the liquid being pumped). It shows that at 250 GPM, the required NPSH of 20 ft, will equal the available NPSH of 20 ft. Therefore, at a flow rate of 250 GPM, the pump will cavitate. This calculation has neglected frictional losses in the suction line, which should be subtracted from the available NPSH. 

Figure 25.2 Required NPSH increases with flow.

Answer—yes But why Well, the liquid is cooled by 5°F after it leaves the drum. The cooled liquid is not in equilibrium with the vapor in the drum. It has been subcooled by 5°F. This means that the bubble-point liquid has been cooled, without altering its composition. The vapor pressure of the liquid has been reduced. As can be seen in Fig. 25.3, subcooling this particular liquid by 5°F reduces its vapor pressure by about 2 psi. As the specific gravity of the liquid is 0.58, this is equivalent to an increase in the NPSH by 8 ft. Once again, our objective is to increase the flow from 250 to 300 GPM. Figure 25.2 tells us that the required NPSH increases from 20 to 26 ft. However, when we subcool the liquid by 5°F, the available NPSH increases from 20 to 28 ft. As the available NPSH now exceeds the required NPSH by 2 ft, the flow can be increased without risk of pump cavitation. 

The most likely explanation for this head loss of 7 ft is frictional loss in the suction line. This reduces the available NPSH from 46 to 39 ft. But this is still a lot more available NPSH than the 14 ft of required NPSH needed to pump 110 GPM. 

This matches the required NPSH, at a flow of 110 GPM, so the pump cavitates. But it still seems as if I am missing at least half of the 46 ft of liquid head to the pump. Where is it ... 

Sump pumps can draw water up from levels as much as 30 ft below the pump s suction. But do such pumps require NPSH Absolutely All centrifugal pumps have some NPSH requirements. What, then, is the available NPSH to the sump pump shown in Fig. 25.8 ... 

This 11.5 ft is the NPSH available to the pump. The pump itself requires only 6 ft of NPSH to pump 1200 GPM of water. Hence, even though the pump s suction is 9 ft above the water in the sump, the available NPSH is twice the required NPSH. 

Recommendations also are made by the Hydraulic Institute of suction specific speeds for multistage boiler feed pumps, with S = 7900 for single suction and 5 = 6660 for double suction. Thus the required NPSH can be found by rearrangement of Eq. (7.14) as... 

For common fluids other than water, the required NPSH usually is lower than for cold water some data are shown in Figure 7.16. 

Manufacturers performance curves, such as those in Fig. 10, contain a great deal of useful information. Actual average head-capacity curves are shown for a number of impeller diameters. Also superimposed on these head curves are curves of constant efficiency. A third set of cur ves superimposed on the head curves are the NPSH requirement curves (dashed line in Fig. 10), which indicate the required NPSH at any given condition of operation. A fourth set of curves sometimes included are the BHP (brake horsepower) curves. BHP is the actual horsepower calculated in the previous HGL method illustration. It is the HHP divided by the effciency. 

The required NPSH, (NPSH)r, is specified by the punqi manufacturer, and the available NPSH, (NPSH)a, is determined by the design of the pun suction piping. To prevent cavitation the available NPSH must be equal to or greater than the required NPSH. 

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